My TWR is over 2 and I can't lift off from Kerbin.

[MickB]

I'm testing out designs for a 2-Kerbal resource mining lander for Minmus, and even though my TWR for the base of the craft on Kerbin is over 2 I can't lift off. I'm using a stock Poodle engine and the craft I'm testing is 12.5 tons. Both Mech Jeb and Kerbal Engineer are telling me my TWR is 2.01 on Kerbin, but when I launch at full thrust it just falls to the ground. Here's a picture of what I'm seeing. 

http://imgur.com/a/xMmpy 

As you can tell I have quite a few mods installed so I don't know if that has anything to do with it, but I haven't had any kind of issue like this before. I've made it to orbit with a TWR as low as 1.12 in the first stage, and this thing just falls straight down.

I know for a fact I'd have no trouble easily landing and taking off from Minmus with this thing, but I want to know where my understanding with this is off. What am I missing here?

 

 

Edited July 23, 2016 by MickB

[MickB]

Ok, nevermind now I see the huge difference between TWR and SLT now. I hadn't ever even looked at SLT before. I always assumed TWR was calculated from sea-level thrust and weight. At sea-level gravitational acceleration is 9.81 m/s², so if my craft is 12,652 kg, that's a force of 124.12 kN. If the sea-level thrust is 64.286 kN that's the .52 SLT I'm seeing. 

Hopefully this helps someone else. I just can't ever remember seeing that much of a discrepancy between TWR and inability to even lift off. 

[Red Iron Crown]

For what it's worth, the engine being used (the Poodle) is one of the absolute worst for atmospheric performance while being one of the best for vacuum performace, that's why the discrepancy is so large for this craft.

[Zhetaan]

Since you figured it out, I'll add for anyone else who doesn't know that when lifting off from Kerbin, be certain to click the 'Atmospheric' button in Kerbal Engineer.  That recalculates thrust and delta-v with the sea-level values.  The reason that button exists is so you can see how your upper stages will perform once they are in space, and then easily convert back and see how your main lifters will do on the pad.

[RobFalcon]

I've seen that button, but I never thought to actually try using it in a good year-or-so of playing...

Thanks for the pointer, Zhetaan!

[Wallygator]

it would be nice if there were an additional column or two in the KER display such that you would not have to click that atmo button (or so that button would not have to exist).

[Red Iron Crown]

4 hours ago, Wallygator said:

it would be nice if there were an additional column or two in the KER display such that you would not have to click that atmo button (or so that button would not have to exist).

I feel the opposite, you'd need three columns at least and that would make the window very large and/or busy. I don't need SL data for vacuum stages and I don't need vacuum data for early ascent stages, KER's presentation works very well for me (though I was very pleased when the Atmo/Vac button was added to compact mode).

[Wallygator]

3 minutes ago, Red Iron Crown said:

I feel the opposite, you'd need three columns at least and that would make the window very large and/or busy. I don't need SL data for vacuum stages and I don't need vacuum data for early ascent stages, KER's presentation works very well for me (though I was very pleased when the Atmo/Vac button was added to compact mode).

Fair enough.

It just means (to me) that the UI for KER could sustain a bit more structured use case analysis.

EDIT: Regardless... it works for me, maybe not so much for others.

Edited July 24, 2016 by Wallygator

[Zhetaan]

17 hours ago, Wallygator said:

It just means (to me) that the UI for KER could sustain a bit more structured use case analysis.

In fairness, there's a reason why it doesn't do that.  Keep in mind that KER doesn't know what you're building, so it has to account for all possibilities.  If you click the 'Atmospheric' button, you'll see a couple of sliders appear at the bottom of the display.  You can use these to adjust the calculation's reference altitude and speed, though they default to sea level and stopped.  Both are relevant to, and were added for the sake of, airbreathing engines.  Airbreathers' thrust and efficiency change with both altitude and speed, and sea-level/stopped only pertains to take-off--i.e., the first ten seconds of the mission.  It's useful, but not enough to justify a permanent display when you account for the sliders' indefinite variability.  (By contrast, MechJeb does have both vacuum and sea-level columns, but it does not, as I recall, have any sliders for altitude.)

The altitude slider is still useful for figuring out middle stages.  It is also useful for defining (or refining) the capabilities of certain kinds of special rockets:  for example, when you have an Eve return vehicle that cannot make it back from sea level but is orbit-capable if launched from a mountaintop (don't overshoot the LZ!), or when you want to make a Jooldiving science return vessel and want to know just how deep it can go and still come back up again (yes, Jeb, we really need that low-atmosphere EVA report!), the altitude slider can tell you quite a lot.

Edited July 25, 2016 by Zhetaan

[Wallygator]

15 hours ago, Zhetaan said:

In fairness, there's a reason why it doesn't do that.  Keep in mind that KER doesn't know what you're building, so it has to account for all possibilities.  If you click the 'Atmospheric' button, you'll see a couple of sliders appear at the bottom of the display.  You can use these to adjust the calculation's reference altitude and speed, though they default to sea level and stopped.  Both are relevant to, and were added for the sake of, airbreathing engines.  Airbreathers' thrust and efficiency change with both altitude and speed, and sea-level/stopped only pertains to take-off--i.e., the first ten seconds of the mission.  It's useful, but not enough to justify a permanent display when you account for the sliders' indefinite variability.  (By contrast, MechJeb does have both vacuum and sea-level columns, but it does not, as I recall, have any sliders for altitude.)

The altitude slider is still useful for figuring out middle stages.  It is also useful for defining (or refining) the capabilities of certain kinds of special rockets:  for example, when you have an Eve return vehicle that cannot make it back from sea level but is orbit-capable if launched from a mountaintop (don't overshoot the LZ!), or when you want to make a Jooldiving science return vessel and want to know just how deep it can go and still come back up again (yes, Jeb, we really need that low-atmosphere EVA report!), the altitude slider can tell you quite a lot.

Seems to me we are talking cross-purposes on this (and apologies as now this is diverting away from the original poster's question (back to that in a minute...)

1) I get everything you say above and have no reason to disagree - facts are facts - assuming they are.

2) My personal opinion is that the user functionality and display approach (in the VAB/SPH)  for the KER UI (not the computational engine) might benefit from further design thought - or user configurability similar (but obviously not the same) to the flight and map interface. Additionally, if the KER engine has to make so many assumptions, then perhaps the user could be allowed to input certain specifics about the vehicle being designed in a manner (via structured input) so that the KER engine might better calculate the performance --> Give the user more control if possible.  Also, I;m not convinced it requires more "Columns", but definitely an efficient information display approach.

3) These are my early morning pre-coffee thoughts - not requirements.  ;-)

Now back to @MickB's initial post...  and the very very sad news that there is no escaping it (or Kerbin for that matter), a poodle is basically just an air circulator at sea level.  Sorry.

If there was ever a truer moment for the "MOAR boosters" meme it's now.

 

[sarbian]

Just use MJ. You can show/hide any columns.

[Red Iron Crown]

3 hours ago, sarbian said:

Just use MJ. You can show/hide any columns.

How can I find the TWR of a stage at 5000m altitude with MJ, though? As far as I can see it's sea level or vacuum with no in between.

[Zhetaan]

5 hours ago, Wallygator said:

[...] A poodle is basically just an air circulator at sea level.  Sorry.

You're forgetting that it doubles as a BBQ when at low power--it's right there in the description!

Anyway, yes, @MickB, you'll want more boosters.  You have a decent (though not aerodynamic; put an adapter on it) upper stage, but it needs help to get to space where it can be useful.

@Wallygator, with respect to your other point, I also have no specific disagreements with anything you said, though I share RIC's concerns about screen clutter and information overload.  I think the crux of our differing interpretations comes from the fact that though I consider KER an essential informational tool, if you want to really get the most out of your rocket, there's a mod for that.

And @sarbian, that mod is usually MechJeb.

[sarbian]

39 minutes ago, Red Iron Crown said:

How can I find the TWR of a stage at 5000m altitude with MJ, though? As far as I can see it's sea level or vacuum with no in between.

I clearly need to add something tonight